Polyethylene terephthalate (PET) polymers are widely used in the packaging industry. PET has excellent mechanical properties as well as optical properties such as high transparency and a high barrier.
Meanwhile, biaxially oriented containers, e.g. bottles, made from PET are widely accepted by customers of the beverage industry. Common sizes for PET bottles range from 0.75 l to 2 l (common US sizes are 20 fl. oz. and 24 fl. oz.). Recently smaller beverage bottles (below 1 l, especially 0.6 l and below) have enjoyed increasing popularity. Such smaller bottles were manufactured from the same PET as the larger bottles, simply by using shorter and thicker parisons. Yet, there are disadvantages associated with the use of the same PET as for larger bottles and just miniaturized parisons.
For the production of beverage bottles, it is important that the polymer is well oriented during stretch-blow molding. Proper orientation results in uniform material distribution in most areas of the bottle and in good physical properties like gas barrier. In particular the production of small bottles with known commercial PET needs short parisons with a thick sidewall. This parison design is necessary in order to achieve the proper orientation of standard PET. A proper orientation means that the area stretch ratio which corresponds to the ratio of a given (marked) area on the stretched bottle (called: bubble) surface to the corresponding area on the surface of the unstretched parison should be about 12.5. In the field of bottle making this area stretch ratio is called ‘natural stretch ratio’ (NSR).
The NSR can be determined in a free-blowing experiment. Free-blowing of thermoplastics, in particular PET and PET copolymers, is a well known technique used to obtain empirical data on the stretching behavior of a particular resin formulation. The method of free blowing PET parisons is described in “Blow Molding Handbook”, edited by Donald V. Rosato, Dominick V. Rosato, Munich 1989. The term “free-blowing” means that a parison is blow-molded without using a mold. Free-blowing a bottle from a parison involves heating the parison to a temperature above its glass transition temperature and then expanding the parison outside of a mold so that it is free to expand without restriction until the onset of strain hardening. Strain hardening can be detected in a stress-strain curve as an upswing in stress following the flow plateau. To a large extent the strain hardening is associated with molecular ordering processes in the resin. Parameters, which exhibit a strong influence on the onset of strain hardening, are molecular weight, the rate of deformation, temperature of the parison and the amount of modifier. If the blow pressure and heating of the parison is properly set for a given parison, it will continue to expand until all of the PET is oriented to the point that stretching will stop at about the natural stretch ratio, or slightly beyond. The outer marked area of the bubble can be converted to a stretch ratio by dividing this marked area by the corresponding outer marked area of the parison.
During injection molding some reduction of the intrinsic viscosity (IV) occurs, and as a consequence the determined NSR is higher, compared to the NSR with no IV reduction. For better comparison of resin properties, the NSR can be calculated for each resin composition. This avoids the influence of process conditions of injection molding on the determined NSR value.
The disadvantage of using known commercial PET in a parison design with a thick sidewall is that a long cooling time is required during injection molding in order to avoid crystallization. A further reduction in size of the parison is limited by the sidewall thickness. If the sidewall thickness is too large, crystallization cannot be prevented during cooling after injection molding.
Thus, in order to avoiding crystallization during injection molding and to improve the strain hardening of known commercial PET, one skilled in the art would probably either add a modifier to the PET or—if already present—try to adjust the amounts of such modifiers. However, common modifiers such as isophthalic acid (IPA), cyclohexanedimethanol (CHDM) or diethylene glycol (DEG) tend to shift the onset of strain hardening to higher stretch values which corresponds to an increase in the NSR, which is disadvantageous. The only commonly known way to reduce the NSR is by way of increasing the molecular weight (i.e. the intrinsic viscosity [IV]) of the PET. Yet, the molecular weight cannot be increased to an extent that would offset the negative influence of the modifier and at the same time decrease the NSR to a sufficiently low value.
A further problem associated with common commercial bottle resins is the high DEG content. The high DEG level in common commercial PET helps to prevent crystallization; on the other hand, the high DEG level makes it impossible to manufacture economically small size polyester bottles for various reasons.
Yet another problem in the manufacture of PET bottles is the crystallization rate of the resin. If the crystallization rate is too high, the process window becomes too narrow. An economic manufacture of small bottles requires, that the crystallization rate is slow. However, some known common commercial polyesters have too high crystallization rates.
Thus, there is still a need for an improved PET resin that is specifically adapted for making containers, and in particular bottles.